As is well known, electrostatographic printers and copiers form toner images on a primary imaging member, transfer the toner images onto a receiver and fuse the toner images to the receiver. In practice, the primary imaging member has a photoconductor surface on which the toner is applied by the sequential steps of uniformly charging the photoconductor exposing the uniformly charged photoconductor to a pattern of light that causes a portion of the uniform charge on the photoconductor to discharge leaving a latent electrostatic image on the photoconductor.
The latent electrostatic image is then exposed to charged toner particles. Electrostatic fields between the primary imaging member and a surface carrying the developer to the exposure window cause the charged toner particles to transfer onto to the primary imaging member according to the pattern and intensity of the electrostatic latent electrostatic image on the photoconductor. The toner image formed on the photoreceptor is then transferred to a receiver by pressing the receiver and the toner image against each other. It is generally preferred to simultaneously apply an electrostatic field to urge the toner particles to the receiver while pressing the receiver against the toner image-bearing primary imaging member.
In some electrophotographic systems the transfer of the toner image is made directly from the primary imaging member to the receiver, however in other electrophotographic systems, the toner image is first transferred from the primary imaging member to an intermediate transfer member and the toner image is subsequently transferred from the intermediate transfer member to a final receiver. The toned receiver is then moved to a fusing station where the toner image is fused to the receiver by heat and/or pressure.
The toner used in electrostatographic systems often takes the form of pigmented thermoplastic particles. In most electrostatographic systems, a process known as tribocharging is used to impart a charge on the pigmented thermoplastic particles. For example, an electrostatographic system that uses a two part developer having toner particles that are mixed with and carried by somewhat larger particles of magnetic material the tribocharging process is performed by mixing the toner particles and magnetic material together. During mixing the magnetic carrier particles interact with the toner particles to impart a generally uniform level of charge on the toner particles so that the toner particles will transfer to the primary imaging member in proportion to of the latent charge image on the photoreceptor.
Thus, it will be appreciated that, as multiple charge images are developed in this manner, toner particles are continuously depleted from the two part developer and that the two part developer must be replenished with fresh toner from time-to-time in order to maintain a concentration of charged toner necessary to provide desired density levels of toner on the primary imaging member. Accordingly, such replenishment toner must be mixed into the developer both to tribocharge the replenishment toner and to provide at least a minimum concentration of charged toner for development. In an electrophotographic printer, the task of mixing toner with carrier to tribocharge the toner and to provide at least a minimum toner concentration is performed by what is known as a development station.
In many electrostatographic printers, the replenishment toner is supplied to the development station from a toner supply bottle that is mounted upside-down i.e., with its mouth facing downward, at one end of the image-development apparatus. Under the force of gravity, toner accumulates at the bottle mouth and a metering device, positioned adjacent the bottle mouth, operates to meter sufficient toner to the developer mix to compensate for the toner lost as a result of image development. Usually, the toner-metering device operates under the control of a toner concentration monitor that continuously senses the ratio of toner to carrier particles in the development mix.
In a typical development station, a housing comprises a sump that contains the developer. The developer is fed to a toning roller that transports the developer into close proximity to the primary imaging member. After toning the primary imaging member, the depleted developer is stripped from the toning roller and transported back into the sump, where it is mixed with fresh developer and, when necessary, the developer is replenished with additional toner to replace the toner that had been deposited onto the primary imaging member. The replenishment toner is introduced into the recirculating developer path and mixed therewith to ensure a uniform toner concentration throughout the developer. To accomplish the mixing, replenishment, feeding and stripping of the development roller, at least one auger is used to advance and to optionally mix any replenishment toner into the developer as the developer is moved through the development system.
The augers used in a development station typically comprise a shaft and have one or more flights of ribbons. The developer exerts significant drag on the augers during rotation. Accordingly, high torques are applied to the augers to overcome this drag.
Another problem caused by the drag exerted by the developer on an auger is that this drag can cause the auger to flex perpendicular to an axis of rotation. This flexing perpendicular to an axis of rotation can cause pinch points with side walls of the chamber or housing of a development station within which the auger is located wherein the developer can be compressed between the auger and the chamber walls. This flexing further increases drag on the auger and can cause agglomerates to form in the developer. This flexing of the auger can also cause another type of drag that occurs when the auger flexes to an extent that allows the auger to rub against side walls of the chamber or housing of a development station within which the auger is located.
It will be appreciated therefore that while it is advantageous to be able to make small, light development stations it is often necessary to make augers larger to accommodate the drag forces when light weight materials are used for auger fabrication, for example, the Xerox 7500 printer sold by Xerox Corp., Rochester, N.Y., USA uses a low density plastic material to fabricate an auger with a relatively large shaft and auger. However, as the size of an auger increases, and, in particular, as the radius or diameter of the auger shaft increases, the auger itself occupies a larger volume of the development station, typically requiring a concomitant increase in the volume of the development station itself. Further, it will be appreciated that when augers are made larger, the size, cost and power of the equipment used to control operation of the auger will increase. Accordingly, the amount of space occupied by a development station that uses such an auger and control equipment can be quite large.
Conversely, smaller stations can be made using comparatively dense materials such as metals to fabricate the augers for example, the RICOH C6000 printer sold by Ricoh, Japan, uses a metal auger. This creates smaller but heavier development stations and requires more complex and costly auger fabrication techniques.
Further, it is known that in some development stations, the task of ensuring that the desired mixing of replenishment toner and developer can be problematic. In such stations, the mixing and transport are often enhanced using paddles. Such paddles increase drag, add cost to an auger, require an increase in the shaft size of the auger, and can create additional pinch points which further increase drag.
Yet another problem created by the drag is that the drag creates loads that cause the auger to translate backwards and forwards along its axis of rotation during rotation. This can also create agglomerates and set up waves of different concentrations of developer flow within a developer during exposure that will result in image density variations in a toner image formed during such exposure.
What is needed in the art therefore are new development stations and methods for operating development stations that allow for smaller equipment size while providing a consistent amount of developer to the primary imaging member and that can more effectively deal with the problems created by toner and developer drag on an auger.